Defining spectroscopic features of heteroannulenic antiaromatic porphyrinoids

A new set of spectroscopic tools is proposed that may be used to distinguish antiaromatic compounds from their corresponding aromatic congeners. This prediction is based on a detailed analysis of the optical and photophysical properties of a matched set of expanded porphyrins. In particular, the antiaromatic porphyrinoids having [4n] -electrons within their conjugation pathway exhibit distinct photophysical features that differ dramatically from what is observed for the corresponding aromatic congeners. The clear diagnostic differences seen between the antiaromatic and aromatic compounds leads us to propose that the spectroscopic methods detailed in this Perspective could emerge as general tools that may be used to characterize the electronic characteristics of complex systems for which a number of potential electronic states can be envisioned on the basis of simple line formulas or analyses of -electron populations.